1. Field of the Invention
The present invention relates to electro-optical imagers for reading a one or two-dimensional symbology, and more particularly, to a fixed position, orientation independent imager utilizing an optical module.
2. Description of Related Art
Optical imaging systems are commonly used to decipher data symbols printed on objects in order to identify the objects. A conventional bar code symbol represents a one-dimensional form of symbology, and comprises a pattern of vertical bars of various widths separated by spaces of various widths. Since the bar and space elements have different light reflecting characteristics, a reader can convert the symbology into an electrical signal by analyzing the light reflected from the symbol. The electrical signal can then be decoded to provide an alphanumeric representation of the symbol which identifies the object. Bar code symbols of this nature are now in common usage in various types of applications, such as inventory control, point of sale identification, or logistical tracking systems.
The bar code reader typically uses a light source that is scanned across the bar code field. Since the bar code symbology is often disposed on the object to be identified, it is desirable for the reader to be included in a hand held or portable device so that the reader can be brought to the object. Light emitting diodes (LEDs) are often utilized to provide the light source due to their light weight and low power requirements. The operator can physically move the LED across the bar code field, such as by use of a light pen. Alternatively, a bar code reader may include movable mirrors that automatically articulate light from a laser or laser diode back and forth at a high rate to scan across the bar code field. The reader may also generate a feedback signal, such as an audible tone, that alerts the operator as to the successful completion of a bar code reading operation.
Since the conventional one-dimensional symbology requires a relatively large amount of space to convey a correspondingly small amount of data, so-called two-dimensional symbologies have been developed. A two-dimensional symbology may comprise a matrix that occupies a uniform amount of space having a generally rectangular or square shape. Instead of bars and spaces, round or square dots disposed at particular rows and columns of the matrix correspond to the characters being conveyed. As a result, a matrix symbology can compress significantly more data into a given volume of space than a conventional one-dimensional bar code. Examples of commercially available two-dimensional symbologies include Code One, Data Matrix, and PDF417.
Portable two-dimensional symbology imagers convert the two-dimensional symbology data into pixel information that is deciphered into the alphanumeric information represented by the symbology data. These two-dimensional imagers can also be utilized to read one-dimensional symbology data, such as conventional bar code. Two-dimensional imagers may utilize charge-coupled device (CCD) technology to convert optical information from the symbology data into an electrical signal, but still require a light source to illuminate the symbology to enable detection by the CCD device. Moreover, to successfully image the symbology, the imager must be capable of determining the rotational orientation of the symbology. A reference symbol disposed adjacent to the symbology data, such as an L or T-shaped character, or other information contained within the symbology, can provide rotational orientation information.
While portable two-dimensional symbology readers are desirable for many applications by giving the operator substantial freedom of movement, they also have several drawbacks, chiefly being their limited power source. Since batteries often represent the heaviest single element of a portable reader, to minimize the reader's total weight manufacturers must invariably make certain trade-offs between battery capacity and device capability. For example, the illuminating capability of the light source may be limited in a manner that restricts the effective range of the device's imaging capability. Also, the processing ability of the device may be intentionally restricted to provide a certain minimum level of capability, which may render the device less sensitive under certain operating conditions.
Conventional fixed mount one-dimensional symbology imagers have been adapted to read two-dimensional symbologies, but these imagers also have significant drawbacks. These fixed mount imagers often utilize an articulated laser that sequentially scans the rows of the matrix symbology; however, such devices are unreliable due to the high number of moving parts. Moreover the devices are entirely orientation dependant, as the operator must align the two-dimensional symbology appropriately with the imager. Similarly, one-dimensional CCD imagers have also been provided that utilize a single one-dimensional row of CCD elements. To scan a two-dimensional matrix symbology, the row of CCD elements must be sequentially moved among the rows of the two-dimensional symbology. As with the articulated laser imagers, one-dimensional CCD imagers are orientation dependent and must be aligned with the symbology to accurately collect the information.
Accordingly, a critical need exists for a low cost, low complexity imager for both one and two-dimensional symbology that permits the communication of a relatively large amount of information while avoiding the drawbacks of conventional portable readers.